1. Field of the Invention
The present invention relates generally to implantable drug delivery devices. More particularly, the invention relates to pressure-sensitive overfill protection systems for preventing overfilling and over pressurization of an implantable drug delivery device.
2. Description of Related Art
Implantable drug delivery systems are in widespread use to provide site-specific and/or sustained delivery of beneficial agents to address adverse patient conditions. Such delivery systems may include implantable infusion pumps, which typically include a pressurized drug reservoir and some form of fluid flow control. One example of an implantable infusion pump is the SYNCHROMED.TM. pump manufactured by Medtronic, Inc. of Minneapolis, Minn.
Typically, implantable drug delivery devices are periodically refilled in situ and percutaneously using a refill device, such as a hypodermic syringe inserted into a refill chamber of the drug delivery device. A common problem related to refilling is the potential for overfilling or over pressurization of the device. Typically, the clinician refilling the device relies upon tactile pressure to sense that the drug reservoir has been filled to capacity. Any additional fluid injected into the device beyond capacity of the reservoir may cause severe damage to the drug delivery device and may cause other adverse consequences.
Prior art overfill prevention devices, such as those described in U.S. Pat. No. 5,158,547, the entire writing of which is incorporated herein by reference, are exemplified by FIG. 1, which is a cross-section of a drug delivery device in the form of an implantable infusion pump 110. Generally, a valve 112 having a rigid valve stem 114 is fastened, usually by welding, to an inner surface 116 of the pump diaphragm or bellows 118. As new drug supply is injected through the septum 120 using a refill device (not shown), the reservoir chamber 122 fills with drug and the surface 116 moves in an expanding direction (downward in FIG. 1). As the reservoir chamber 122 reaches capacity, the valve 112 is pulled into sealing engagement with a valve seat 126, resulting in a detectable increase in pressure in the refill device and preventing overfilling and/or over pressurization of the drug delivery device 110.
Notably, prior art devices are volume-sensitive and rely on the displacement of the bellows surface for actuating an inlet valve. One problem with such devices is that the particular size of the valve stem is dependent on the size of the bellows. Accordingly, when different sized bellows are used in a particular pump configuration, the overfill protection device must be modified to provide for valve closure at the appropriate bellows volume. Another problem with such prior art configurations is that, due to the tolerance stack up, i.e. the overall variation in valve dimensions that results from the dimensions of the various parts that make up the valve, prior art protection systems are typically constructed such that the bellows is slightly underfilled when the valve seats. Thus, the full capacity of the bellows is not utilized.
Another problem with prior art systems is that the over pressurization protection valve stem is welded directly to the bellows surface. In addition to the construction costs associated with this exercise, there is the potential that the bellows, which is typically constructed of a thin metal membrane, may be damaged during the welding or fastening operation.
What is needed is an overfill protection system for implantable drug delivery devices which addresses the aforementioned problems. Specifically, what is needed is a an overfill protection system for an implantable drug delivery device that is pressure-sensitive and which is more economical to manufacture than prior art systems.